System and method for triggering power transfer across an inductive power coupling and non resonant transmission

ABSTRACT

A triggerable power transmitter for power transmission from a primary coil to an inductively coupled secondary coil in a power receiver has a primary coil; a driver for electrically driving the primary coil; a probing coil receives analog signals indicative of resonance properties of the primary coil; analog filters may be used to filter frequencies, and a processor capable of generating digital information in response to the analog signal and determining if said primary coil is coupled to a secondary coil based on the digital information, and triggering power from the primary coil to said secondary coil when said primary coil is inductively coupled to said secondary coil. A resistor may be selectably connected in series with the primary coil and shorted out when power is transmitted.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/531,397, filed Dec. 6, 2013, which in turn is a continuation ofInternational Patent Application No. PCT/IL2013/050379, filed May 2,2013, which is based upon and claims the benefit of U.S. ProvisionalPatent Application Ser. No. 61/642,165, filed May 3, 2012, and Ser. No.61/650,683 filed May 23, 2012, the disclosures of which are herebyincorporated by reference in their entirety herein.

FIELD OF THE DISCLOSURE

The disclosure herein is directed to providing devices, a system andmethod for triggering and controlling power transfer across an inductivepower coupling. The disclosure further provides a communications channelfor the transfer of feedback signals in inductive power transfersystems. More specifically, the present disclosure relates tocoil-to-coil signal transfer in inductive power couplings.

BACKGROUND OF THE INVENTION

Inductive power coupling allows energy to be transferred from a powersupply to an electric load without a wired connection therebetween. Apower supply is wired to a primary coil and an oscillating electricpotential is applied across the primary coil which induces anoscillating magnetic field therearound. The oscillating magnetic fieldmay induce an oscillating electrical current in a secondary coil, placedclose to the primary coil. In this way, electrical energy may betransmitted from the primary coil to the secondary coil byelectromagnetic induction without the two coils being conductivelyconnected.

When electrical energy is transferred from a primary inductor to asecondary inductor, the inductors are said to be inductively coupled. Anelectric load wired in series with such a secondary inductor may drawenergy from the power source wired to the primary inductor when thesecondary inductor is inductively coupled thereto.

For safety, the power supplying side of a conductive couple is generallythe female part, and does not have bare conductive elements protrudingtherefrom. Nevertheless, socket holes are dangerous and children dosometimes manage to insert pencils, pins and other objects into socketholes, sometimes with fatal results. Water can also cause shorting andmay result in electrocution.

It can therefore be safer and more reliable to provide socket-less poweroutlets such as inductive couplers. Inductive power coupling allowsenergy to be transferred from a power supply to an electric load withoutconnecting wires, as detailed hereinabove.

Low power inductive electrical power transmission systems over extendedsurfaces are not new. One such example is described in U.S. Pat. No.7,164,255 to Hui. In Hui's system a planar inductive battery chargingsystem is designed to enable electronic devices to be recharged. Thesystem includes a planar charging module having a charging surface onwhich a device to be recharged is placed. Within the charging module,and parallel to the charging surface, at least one, and preferably anarray of primary windings are provided. These couple energy inductivelyto a secondary winding formed in the device to be recharged. Suchsystems are adequate for charging batteries in that they typicallyprovide a relatively low power inductive coupling. It will beappreciated however, that extended base units such as Hui's chargingsurface which transmit energy continually approximately uniformly overthe whole area of the unit, are not suitable for use with high energysystems.

United States Patent Application 2010/0072825, to Azancot et. al.discloses a system and method for controlling power transfer across aninductive power coupling. The application discloses a signal transfersystem for controlling power transfer across an inductive powercoupling. A transmission circuit associated with an inductive powerreceiver is configured to transmit a control signal to a receptioncircuit associated with an inductive power outlet. The signal transfersystem may be used to regulate the power supplied by the inductivecoupling and to detect the presence of the secondary coil.

It is noted that that the strength of the induced voltage in thesecondary inductor varies according to the oscillating frequency of theelectrical potential provided to the primary inductor. The inducedvoltage is strongest when the oscillating frequency equals the resonantfrequency of the system. The resonant frequency f_(R) depends upon theinductance L and the capacitance C of the system according to theequation

$f_{R} = {\frac{1}{2\pi \sqrt{LC}}.}$

It is further noted that known inductive power transfer systemstypically transmit power at the resonant frequency of the inductivecouple. This can be difficult to maintain as the resonant frequency ofthe system may fluctuate during power transmission, for example inresponse to changing environmental conditions or variations in alignmentbetween primary and secondary coils.

Therefore, inductive transfer systems designed to transmit at resonancerequire tuning mechanisms for maintaining transmission at the resonantfrequency of the system. Tuning may be achieved by adjusting the drivingfrequency to seek resonance. For example, U.S. Pat. No. 6,825,620,titled “Inductively coupled ballast circuit” to Kuennen et al. describesa resonance seeking ballast circuit for inductively providing power to aload. The ballast circuit includes an oscillator, a driver, a switchingcircuit, a resonant tank circuit and a current sensing circuit. Thecurrent sensing circuit provides a current feedback signal to theoscillator that is representative of the current in the resonant tankcircuit. The current feedback signal drives the frequency of the ballastcircuit causing the ballast circuit to seek resonance. The ballastcircuit includes a current limit circuit that is inductively coupled tothe resonant tank circuit. The current limit circuit disables theballast circuit when the current in the ballast circuit exceeds apredetermined threshold or falls outside a predetermined range.

Alternatively, tuning may be achieved by adjusting the characteristicsof the inductive system. For example, U.S. Pat. No. 2,212,414, titled“Adaptive inductive power supply” to Baarman describes a contactlesspower supply which has a dynamically configurable tank circuit poweredby an inverter. The contactless power supply is inductively coupled toone or more loads. The inverter is connected to a DC power source. Whenloads are added or removed from the system, the contactless power supplyis capable of modifying the resonant frequency of the tank circuit, theinverter frequency, the inverter duty cycle or the rail voltage of theDC power source.

Tuning mechanisms such as those described above are necessary in orderto maintain transmission at resonance because resonant transmission ishighly sensitive. At resonance small variations to the system result inlarge changes to the power transferred. A further problem associatedwith resonant transmission is the high transmission voltages involved.At high operating voltages, the capacitors and transistors in thecircuit need to be relatively large.

There is a need for an inductive transfer system with a higher toleranceto environmental fluctuations and variations in inductive coil alignmentand which transmits at low voltages. The present invention addressesthis need.

SUMMARY OF THE INVENTION

It is an aspect of the current disclosure to provide a method oftriggering power transmission in inductively coupled power transmissionsystem comprising: a. waiting a time duration; b. electrically excitinga primary coil in a power transmitter; c. receiving a signal indicativeof resonance properties of the primary coil; d. determining if asecondary coil in a power receiver is inductively coupled to the primarycoil; and e. triggering power transmission from the primary coil to saidsecondary coil if the secondary coil is inductively coupled to theprimary coil, or repeating steps a-d if the secondary coil is notinductively coupled to the primary coil.

In some embodiments exciting of the primary coil in a power transmittercomprises applying a short electric pulse to the primary coil.

In some embodiments determining if the secondary coil is inductivelycoupled to the primary coil comprises determining a change in resonancefrequency of the primary coil.

In some embodiments determining if the secondary coil is inductivelycoupled to said primary coil comprises determining that the change inresonance frequency of said primary coil is a reduction of the resonancefrequency.

In some embodiments determining if the secondary coil is inductivelycoupled to the primary coil comprises determining a change in effectiveinductance of the primary coil.

In some embodiments determining if the secondary coil is inductivelycoupled to the primary coil further comprises determining a change ineffective resistance of the primary coil.

In some embodiments determining if the secondary coil is inductivelycoupled to the primary coil comprises determining a match between:values indicative of effective inductance of the primary coil; andvalues indicative of effective resistance of the primary coil to atleast one set of values associated with the primary coil inductivelycoupled to the secondary coil.

In some embodiments determining if the secondary coil is inductivelycoupled to the primary coil comprises determining a match between:values indicative of effective inductance of the primary coil; andvalues indicative of effective resistance of the primary coil to atleast one set of values in a list of values associated with a primarycoil inductively coupled to a plurality of different types of powerreceivers.

In some embodiments, triggering power transmission from the primary coilto the secondary coil if the secondary coil is inductively coupled tothe primary coil comprises controlling the power transmission accordingto type of a power receiver associated with the matched valuesindicative of effective inductance of the primary coil; and valuesindicative of effective resistance of the primary coil.

In some embodiments repeating steps a-d if the secondary coil is notinductively coupled to the primary coil further comprising issuing awarning if the signal indicative of resonance properties of the primarycoil indicates that an object other than the secondary coil isinductively coupled to the primary coil.

In some embodiments exciting of a primary coil in a power transmittercomprises short duration activation of a driver used for driving theprimary coil during power transmission from the primary coil to thesecondary coil.

In some embodiments exciting of the primary coil in a power transmittercomprises activation of the driver used for driving the primary coilduring power transmission from the primary coil to the secondary coil atpower level significantly reduced compared to power levels used fordriving the primary coil during power transmission.

In some embodiments exciting of the primary coil in the powertransmitter at reduced power level comprising exciting the primary coilat a plurality of frequencies, and determining if the secondary coil inthe power receiver is inductively coupled to the primary coil comprisesassessing frequency response of the primary coil.

It is another aspect of the disclosure to provide a triggerable powertransmitter for power transmission from a primary coil in the powertransmitter to an inductively coupled secondary coil in a power receivercomprising: a primary coil, capable of being inductively coupled to asecondary coil in a power receiver; a driver, capable of electricallydriving the primary coil; a front end, capable of receiving analogsignal indicative of resonance properties of the primary coil andcapable of generating digital information in response to the analogsignal; and a processor, receiving the digital information and capableof: determining if the primary coil is coupled to a secondary coil basedon the digital information, and controlling the driver to transmit powerfrom the primary coil to the secondary coil when the primary coil isinductively coupled to the secondary coil.

In some embodiments the front end is connected to the primary coil.

In some embodiments the transmitter further comprises a probing coil;the probing coil is capable of providing the front end with analogsignal indicative of resonance properties of the primary coil.

In some embodiments the transmitter further comprises a resistor, placedin series to primary coil and capable of providing the front end withanalog signal indicative of resonance properties of the primary coil.

In some embodiments the transmitter further comprises a switch placed inparallel to the resistor and capable of shorting out the resistor whenpower is transmitted from the primary coil to the secondary coil.

In some embodiments the transmitter further comprises a plurality ofalignment coils capable of providing signals indicative of misalignmentof the secondary coil in relation to the primary coil.

It is noted that some embodiments of the present disclosure are furtherdirected towards providing an inductive power transfer system adapted totransmit power at a non-resonant frequency comprising at least oneinductive power outlet comprising at least one primary inductive coilwired to a power supply via a driver, wherein the at least one primaryinductive coil associated with an inductive power receiver; and a remotesecondary unit comprising at least one secondary inductive coil wired toan electric load, the at least one secondary inductive coil associatedwith an inductive power transmitter; wherein the at least one primaryinductive coil is configured to form an inductive couple with the atleast one secondary inductive coil and wherein the driver is configuredto provide a driving voltage across the primary inductive coil, thedriving voltage oscillating at a transmission frequency significantlydifferent from the resonant frequency of the inductive couple.Optionally, the driver comprises a switching unit for intermittentlyconnecting the primary inductive coil to the power supply.

The inductive power transfer system further comprising an inductivefeedback channel for transferring signals concurrently withuninterrupted inductive power transfer between the inductive powertransmitter and the inductive power receiver.

Where appropriate, the transmission frequency may lie within a range inwhich induced voltage varies approximately linearly with frequency.Optionally, the driver is configured to adjust the transmissionfrequency in response to feedback signals.

Optionally, the inductive power outlet comprises a signal detectoradapted to detect a first signal and a second signal, and the driver isconfigured to: increase the transmission frequency when the first signalis detected by the detector, and decrease the transmission frequencywhen the second signal is detected by the detector. The feedback signalsgenerally carry data pertaining to the operational parameters of theelectric load. Operational parameters are selected from the groupcomprising: required operating voltage for the electric load; requiredoperating current for the electric load; required operating temperaturefor the electric load; required operating power for the electric load;measured operating voltage for the electric load; measured operatingcurrent for the electric load; measured operating temperature for theelectric load; measured operating power for the electric load; powerdelivered to the primary inductive coil; power received by the secondaryinductive coil, and a user identification code. Optionally, the detectoris selected from the list comprising optical detectors, radio receivers,audio detectors and voltage peak detectors.

Optionally, the driver further comprises a voltage monitor formonitoring the amplitude of a primary voltage across the primary coil.Optionally, the voltage monitor is configured to detect significantincreases in primary voltage.

In some cases, the driving voltage oscillating at a transmissionfrequency higher than the resonant frequency of the inductive couple,wherein the primary inductive coil is further wired to a receptioncircuit comprising a voltage monitor for monitoring the amplitude of aprimary voltage across the primary coil, and the secondary inductivecoil is further wired to a transmission circuit for connecting at leastone electric element to the secondary inductive coil thereby alteringthe resonant frequency such that a control signal may be transferredfrom the transmission circuit to the reception circuit. Variously, theresonance altering electric element may include a capacitor configuredto connect to the secondary inductive coil either in parallel or seriesand thereby to effectively increase or decrease the resonant frequencyof the system. Alternatively or additionally, the resonance alteringelectric element may include an inductor configured to connect to thesecondary inductive coil either in parallel or series and thereby toeffectively increase or decrease the resonant frequency of the system.Still further, the resonance altering electric element may be dampingelement such as a resistor, capacitor, inductor or the like operable toconnect to the secondary inductor thereby damping the resonance andeffectively increasing the resonant frequency of the system.Alternatively damping may be reduced, for example by adding dampingelements such as resistors in parallel to extant damping elements ordisconnecting extant damping elements from the secondary inductor.

Optionally, the secondary inductive coil is wired to two inputs of abridge rectifier and the electric load is wired to two outputs of thebridge rectifier wherein the transmission circuit is wired to one inputof the bridge rectifier and one output of the bridge rectifier.Typically, the transmission circuit further comprises a modulator formodulating a bit-rate signal with an input signal to create a modulatedsignal and a switch for intermittently connecting the electrical elementto the secondary inductive coil according to the modulated signal.Optionally, the voltage monitor further comprises a correlator forcross-correlating the amplitude of the primary voltage with the bit-ratesignal for producing an output signal.

In certain embodiments, the control signal is usable for transferring afeedback signal from the secondary inductive coil to the primaryinductive coil for regulating power transfer across an inductive powercoupling. The driver may be configured to adjust the transmissionfrequency in response to the feedback signals. Accordingly, the systemmay be adapted to transfer a first signal and a second signal, and thedriver is configured to: increase the transmission frequency when thefirst signal is received by the receiver, and decrease the transmissionfrequency when the second signal is received by the receiver.

In an alternative embodiment of the inductive power transfer system, theinductive feedback channel may comprise a transmission circuitincorporated in the secondary unit and a receiving circuit incorporatedin the inductive power outlet.

The inductive feedback channel may further comprise at least oneauxiliary coil operable to detect fluctuations in the magnetic field inthe vicinity of the primary inductor and of the at least one secondaryinductor to pick up feedback signals.

Optionally, the at least one auxiliary coil may be connected to thereceiving circuit configured to produce output signals based on themagnetic fluctuations.

Optionally, the output signals may be transmission parameters configuredto adjust transmission frequency.

Additionally or alternatively, the output signals may includeoperational parameters configured to provide data instructions to thereceiving circuit.

Optionally, the auxiliary coil comprises an external coil to providefiltering.

Where required, one or more auxiliary coils may be incorporated into thetransmission circuit.

Variously, the system may be incorporated into at least one applicationselected from a group consisting of: inductive chargers, inductive poweradaptors, power tools, kitchen appliances, bathroom appliances,computers, media players, office equipment, implanted devices, pacemakers, trackers and RFID tags inductive chargers, inductive poweradaptors.

Furthermore the current disclosure teaches a method for regulating powertransmission inductive from a primary inductive coil, wired to a powersupply via a driver, to a secondary inductive coil, wired to an electricload, the method comprising the following steps: (a)—providing anoscillating voltage to the primary inductive coil at an initialtransmission frequency ft which is substantially different from theresonant frequency f_(R) of the system; (b)—inducing a secondary voltagein the secondary inductive coil; (c)—monitoring power received by theelectric load; (d)—sending a feedback signal when the monitored powerdeviates from a predetermined range; (e)—the driver receiving thefeedback signal; (f)—the driver adjusting the transmission frequency;and (g)—repeating steps (b)-(f).

Optionally, step (d) further comprises: (d1) sending a feedback signalof a first type S_(a) to the driver, whenever the power drops below apredetermined lower threshold, and (d2) sending a feedback signal of asecond type S_(b) to the driver, whenever the power exceeds apredetermined upper threshold.

Optionally, the initial transmission frequency f_(t) is higher than theresonant frequency f_(R) and step (f) further comprises: (f1) the driverreducing the transmission frequency by an incremental value −δf₁ whenthe received feedback signal is of the first type S_(a), and (f2) thedriver increasing the transmission frequency by an incremental value+δf₂ when the received feedback signal is of the second type S_(b).

In still other embodiments, the invention is directed to teachinganother method for transferring a signal from a secondary inductive coilto a primary inductive coil of an inductive power transfer system, saidmethod comprising the following steps: Step (i)—connecting the primaryinductive coil to a voltage monitor for monitoring the amplitude of aprimary voltage across the primary coil; Step (ii)—connecting thesecondary inductive coil to a transmission circuit for selectivelyincreasing the resonant frequency of the inductive power transfersystem; Step (iii)—providing an oscillating voltage to the primaryinductive coil at an initial transmission frequency higher than theresonant frequency thereby inducing a voltage in the secondary inductivecoil; Step (iv)—using the transmission circuit to modulate a bit-ratesignal with the input signal to create a modulated signal and connectinga electrical element to the secondary inductive coil intermittentlyaccording to the modulated signal, and Step (v)—using the voltagemonitor to cross-correlate the amplitude of the primary voltage with thebit-rate signal for producing an output signal.

Variously, the resonance altering electrical element may include acapacitor configured to connect to the secondary inductive coil eitherin parallel or series and thereby to effectively increase or decreasethe resonant frequency of the system. Alternatively or additionally, theresonance altering electrical element may include an inductor configuredto connect to the secondary inductive coil either in parallel or seriesand thereby to effectively increase or decrease the resonant frequencyof the system. Still further, the resonance altering electrical elementmay be damping element such as a resistor, capacitor, inductor or thelike operable to connect to the secondary inductor thereby damping theresonance and effectively increasing the resonant frequency of thesystem. Alternatively damping may be reduced, for example by addingdamping elements such as resistors in parallel to extant dampingelements or disconnecting extant damping elements from the secondaryinductor.

Unless otherwise defined, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs. Although methods and materialssimilar or equivalent to those described herein can be used in thepractice or testing of the present invention, suitable methods andmaterials are described below. In case of conflict, the patentspecification, including definitions, will control. In addition, thematerials, methods, and examples are illustrative only and not intendedto be limiting.

BRIEF DESCRIPTION OF THE FIGURES

For a better understanding of the invention and to show how it may becarried into effect, reference will now be made, purely by way ofexample, to the accompanying drawings.

With specific reference now to the drawings in detail, it is stressedthat the particulars shown are by way of example and for purposes ofillustrative discussion of the preferred embodiments of the presentinvention only, and are presented in the cause of providing what isbelieved to be the most useful and readily understood description of theprinciples and conceptual aspects of the invention. In this regard, noattempt is made to show structural details of the invention in moredetail than is necessary for a fundamental understanding of theinvention; the description taken with the drawings making apparent tothose skilled in the art how the several forms of the invention may beembodied in practice. In the accompanying drawings:

FIG. 1 schematically depicts a block diagram of the main elements of aninductive power transfer system as known in the art.

FIG. 2 schematically depicts a block diagram of the main elements of aninductive power transfer system according to an exemplary embodiment ofthe current invention.

FIG. 3 schematically depicts an electrical circuitry according to anexemplary embodiment of the current invention.

FIG. 4 schematically depicts experimental results from a setupequivalent to circuit 300 wherein front end unit was replaced with anoscilloscope.

FIG. 5A depicts the voltage measured on the primary coil after pulseexcitation where no receiver was placed in proximity to the primarycoil.

FIG. 5B depicts the voltage measured on the primary coil after pulseexcitation where ferromagnetic material was placed in proximity to theprimary coil, representing coupling to an unloaded secondary coil.

FIG. 5C depicts the voltage measured on a primary coil after pulseexcitation where a thin layer of copper was placed in proximity to theprimary coil, representing incorrect placement of an object, orplacement of foreign object on the transmitter.

FIG. 6A schematically depicts a block diagram of the main elements of aninductive power transfer system which uses a series resistor duringdetection of inductive coupling according to an exemplary embodiment ofthe current invention.

FIG. 6B schematically depicts a block diagram of the main elements of aninductive power transfer system which uses a probing coil 520 fordetection of inductive coupling according to an exemplary embodiment ofthe current invention.

FIG. 7 schematically depicts a spatial positioning of a primary coil andthree alignment coils according to an exemplary embodiment of thecurrent invention.

FIG. 8 schematically depicts a method for triggering power transmissionfrom the transmitter to the receiver according to an exemplaryembodiment of the current invention.

FIG. 9 is a block diagram showing the main elements of an inductivepower transfer system with a feedback signal path according toembodiments of the present invention;

FIG. 10 is a graph showing how the amplitude of operational voltage ofan inductive power transfer system varies with transmission frequency;

FIG. 11 is a schematic diagram representing a laptop computer drawingpower from an inductive power outlet;

FIG. 12 is a circuit diagram of an inductive power transfer systemaccording to another embodiment of the invention including a peakdetector for detecting large increases in transmission voltage;

FIG. 13 is a flowchart showing a method for regulating power transfer byvarying the power transmission frequency in an inductive power transfersystem according to a further embodiment of the invention;

FIG. 14 is a block diagram showing the main elements of an inductivepower transfer system with an inductive feedback channel according tostill another embodiment of the present invention;

FIG. 15A is a graph showing how the variation of operational voltagewith transmission frequency is affected by changes in resonant frequencyof the system;

FIG. 15B is a graph showing how the variation of operational voltagewith transmission frequency is affected by changes in resonant frequencyof the system as a result of damping;

FIG. 16A is a circuit diagram of an inductive power transfer systemincluding an inductive feedback channel for providing coil-to-coilsignal transfer concurrently with uninterrupted inductive power transferbetween the coils in accordance with yet another embodiment of theinvention;

FIG. 16B is a circuit diagram of another inductive power transfer systemincluding an inductive feedback channel including an auxiliary pickupcoil for providing signal transfer; and

FIG. 17 is a flowchart showing a method for inductively transferring asignal from the secondary inductive coil to a primary inductive coil ofan inductive power transfer system according to still a furtherembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

As required, detailed embodiments of the present disclosure aredisclosed herein; however, it is to be understood that the disclosedembodiments are provided merely as illustrative of the disclosure thatmay be embodied in various and alternative forms. The figures are notnecessarily to scale; some features may be exaggerated or minimized toshow details of particular components. Therefore, specific structuraland functional details disclosed herein are not to be interpreted aslimiting, but merely as a representative basis for teaching one skilledin the art to variously employ the present disclosure.

Before explaining at least one embodiment of the disclosure in detail,it is to be understood that the disclosure is not necessarily limited inits application to the details set forth in the following description orexemplified by the Examples. The disclosure is capable of otherembodiments or of being practiced or carried out in various ways.

The terms “comprises”, “comprising”, “includes”, “including”, and“having” together with their conjugates mean “including but not limitedto”. The term “consisting of” has the same meaning as “including andlimited to”. The term “consisting essentially of” means that thecomposition, method or structure may include additional ingredients,steps and/or parts, but only if the additional ingredients, steps and/orparts do not materially alter the basic and novel characteristics of theclaimed composition, method or structure.

As used herein, the singular form “a”, “an” and “the” include pluralreferences unless the context clearly dictates otherwise. For example,the term “a compound” or “at least one compound” may include a pluralityof compounds, including mixtures thereof.

Throughout this application, various embodiments of this invention maybe presented in a range format. It should be understood that thedescription in range format is merely for convenience and brevity andshould not be construed as an inflexible limitation on the scope of theinvention. Accordingly, the description of a range should be consideredto have specifically disclosed all the possible sub-ranges as well asindividual numerical values within that range.

It is appreciated that certain features of the invention, which are, forclarity, described in the context of separate embodiments, may also beprovided in combination in a single embodiment. Conversely, variousfeatures of the invention, which are, for brevity, described in thecontext of a single embodiment, may also be provided separately or inany suitable sub-combination or as suitable in any other describedembodiment of the invention. Certain features described in the contextof various embodiments are not to be considered essential features ofthose embodiments, unless the embodiment is inoperative without thoseelements.

In discussion of the various figures described herein below, likenumbers refer to like parts. The drawings are generally not to scale.For clarity, non-essential elements were omitted from some of thedrawing. Reference is now made to the drawings.

FIG. 1 schematically depicts a block diagram of the main elements of aninductive power transfer system 100 as known in the art.

The inductive power coupling 200 consists of a primary inductive coil220 and a secondary inductive coil 260. The primary coil 220 is wired toa power supply 240, typically via a driver 230 which provides theelectronics necessary to drive the primary coil 220. Driving electronicsmay include a switching unit providing a high frequency oscillatingvoltage supply, for example. The secondary coil 260 is wired to anelectric load 280. For drawing clarity, internal details of load 280 arenot seen in this and the following figures. It should be noted that load280 may comprise rectification and other electronic circuitry. Forexample, for battery charging application, load 280 may represent arechargeable battery, an AC to DC rectification, and optionally at leastone of: a DC to DC power conditioning circuits, battery chargingregulation functions, and means of communication with transmitter 210 inorder to regulate power transmission to meet the power requirements ofthe battery charger, and optionally to terminate power transmission whenbattery is fully charged.

When the secondary coil 260 is brought into proximity with the primarycoil 220, the pair of coils forms an inductive couple and power istransferred from the primary coil 220 to the secondary coil 260. In thisway a power transmitter 210 may provide power to an electric receiverdevice 290.

However, it is important to activate the power transmission only whenthe secondary coil 260 is present, and correctly aligned with theprimary coil 220. Activation of power transmission without properplacement of the power receiver 290 may cause electromagnetic wavesradiating from the primary coil 220 to leak into the environment,possibly causing energy loss, electromagnetic interference, and healthrisk.

A sensor 160 is used for detecting the presence of receiver 290 andsignaling processor 150 to activate driver 230 to start activatingprimary coil 220 only when receiver 290 is detected.

It should be noted that typically inductive power transfer system 100may stand idle for much of the time, and is activated infrequently andfor some limited time duration. Such is the case wherein inductive powertransfer system 100 is used for example recharging a cellular phone andthe likes. To reduce energy consumption, the system should remain in lowpower consumption state before detecting the presence of receiver 290.

Processor 150 is optionally used for controlling other functions ofsystem 100, for example it may be used for regulating power transmissionaccording to the power requirements of load 280. Optionally, processor150 further terminates power transmission, for example when arechargeable battery in load 280 is fully charged.

Additionally, it is important that the inductive coupling 200 betweenprimary coil 220 and secondary coil 260 is not interrupted by foreignmaterial that may be placed between the transmitter 210 and the receiver290. For example even a thin layer of conductive material such as metalfoil may absorb enough of the transmitted electrical power to produceheat, thus creating fire hazard.

Several types of sensors 160 may be used, these include for example:magnetic sensors such as Hall Effect sensors that detects changes inmagnetic field caused by magnetic material at the receiver; audiosensors such as a microphone capable of detecting vibration caused byplacing the receiver on the transmitter, capacitive sensors; weightdetection, mechanical switch and others.

FIG. 2 schematically depicts a block diagram of the main elements of aninductive power transfer system 190 according to an exemplary embodimentof the current invention.

It is one system the primary coil 22 may be used as a sensor for sensingthe presence of the receiver 290 instead of using sensor 160.

In the example of FIG. 2, primary coil 220 is connected to front endelectronics 170 which is capable of sensing changes in the inductance ofprimary coil 22 due its inductive coupling 200 to secondary coil 260.

Primary coil 220 may be a part of a resonance circuit having a resonancefrequency according to the inductance and capacitance in said circuit.When the inductance in the circuit changes the resonance frequencychanges accordingly. The change in resonance frequency may be used as anindication that a secondary coil 260 is coupled to primary coil 220,thus triggering power transfer from transmitter 110 to receiver 290.

The new resonance frequency may be used for assessing the quality of thecoupling 200 between primary coil 220 and secondary coil 260. Thisinformation may be used for indication to the user of the system 190 tocorrect poor alignment or incorrect placement of receiver 290.Furthermore, the measured resonant frequency may be used to determinethe required transmission frequency of the system.

FIG. 3 schematically depicts an electrical circuitry 300 according whichmay be used in the power transmission system.

Electrical circuit 300 shows a DC power supply 240 connected to an ACdriver comprising FET switches M1 and M1 and to a resonance circuitcomprising the primary coil 220 and capacitors C1 and C2.

In power transmission mode, switches M1 and M2 are sequentiallyactivated by gating signals 151 and 152 respectively, causing AC currentto flow through primary coil 220 which induces generation of voltage inthe inductively coupled secondary coil 260. For drawing clarity elementsnot needed for the understanding of the operation of the circuit wereomitted.

Primary coil 220 is connected to front end 170, for example via line 174connected to terminal 630 of coil 220 which senses the voltage on thecoil and provides processor 150 with signals 172 indicative of thechanges in electrical properties of the resonance circuit caused by theproximity of the receiver 290. Additionally or alternatively, front end170 may be connected to the other terminal 631 of coil 220.Alternatively, front end 170 may be connected to both terminals, sensingthe voltage difference between terminals 630 and 631. Alternatively,front end 170 may be connected to some partial number of the windings ofprimary coil 220.

FIG. 4 schematically depicts experimental results from a setupequivalent to circuit 300 wherein front end 170 was replaced with anoscilloscope.

Graph 400 shows two traces: Trace 410 shows the voltage on gating signal151 while trace 450 is the voltage 174 on primary coil 220.

At point 412 of trace 410, gate signal 151 goes from “zero” to “one”closing switch M1 while switch M2 is open. This causes a rise 452 involtage 174 as indicated by the rise of trace 450. The current onprimary coil 220 start to oscillate at the resonance frequency given bythe effective values of the inductive L1 of coil 220 and the equivalentcapacitance (approximately given by the sum of capacitance C1+C2, butmay be effected by other stray and real capacitance not seen in thisfigure) in the resonance circuit. In the absence of load, theoscillations decay due to losses in the resonance circuit, for exampledue to Ohmic resistance of coil 220 and energy dissipation due toelectromagnetic waves radiated by the coil.

At point 422 of trace 410, gate signal 151 goes from “one” to “zero”opening switch M1 while switch M2 closes. This causes a fall 462 involtage 174 as indicated by the fall of trace 450.

It should be noted that the oscillations of the current in primary coil220 are caused by the pulsed nature of the excitation caused by theclosing and opening of switches M1 and M2. Resonance frequency may bedetermined for example by measuring the time interval 470 taken for oneoscillation. The decay envelop 480 of oscillations of trace 450 isindicative of the losses in the resonance circuit.

After a pulsed excitation, and in absence of appreciable load, thecurrent in the primary coil oscillate at the resonance frequency f givenby:

$\begin{matrix}{f = {\frac{1}{2\pi}\sqrt{\frac{1}{LC} - \left( \frac{R}{2L} \right)^{2}}}} & (1)\end{matrix}$

wherein L is the inductance, C is the capacitance and R is theresistance in the circuit.Since the resistance is low, the resonance frequency may be approximatedby:

$\begin{matrix}{f \approx {\frac{1}{2\pi}\sqrt{\frac{1}{LC}}}} & (2)\end{matrix}$

The resonance frequency f in hertz, may be measured from graph 450 bymeasuring the time interval T 470 and calculating:

$\begin{matrix}{f = \frac{1}{T}} & (3)\end{matrix}$

By combining equations (2) and (3) we can estimate the inductance L by:

$\begin{matrix}{L = \frac{T^{2}}{4C\; \pi^{2}}} & (4)\end{matrix}$

The envelop 480 of trace 450 decays exponentially with damping factor αgiven by:

$\begin{matrix}{\alpha = \frac{R}{2L}} & (5)\end{matrix}$

such that that trace 450 is given approximately by:

sin(2πft)  (6)

where t is the time. Thus, the resistance R may be estimated by:

R=2Lα  (7)

FIGS. 5A, 5B and 5C depict voltages measured on a primary coil 220 afterpulse excitation in different conditions, wherein:

FIG. 5A depicts the voltage measured on a primary coil 220 after pulseexcitation where no receiver was placed in proximity to the primarycoil.

FIG. 5B depicts the voltage measured on a primary coil 220 after pulseexcitation where ferromagnetic material was placed in proximity to theprimary coil, representing coupling to an unloaded secondary coil 260.

FIG. 5C depicts the voltage measured on a primary coil 220 after pulseexcitation where a thin layer of copper was placed in proximity to theprimary coil, representing incorrect placement of an object, orplacement of a foreign object on the transmitter.

From FIG. 5A the natural resonance frequency of the transmitter circuitmay be estimated as f˜190.1 kilohertz, and a decay to half the amplitudeis in ˜25 microseconds.

From FIG. 5B we can see that the resonance frequency of the transmittercircuit decreased to f˜141.2 kilohertz, and a decay to half theamplitude is in ˜35 microseconds.

This is caused primarily by the increase in coil inductance due tocoupling with the ferromagnetic material. It should be noted that inactual systems, secondary coil 260 is fitted with a ferromagnetic coreto increase the coupling to the primary coil 220, thus ensuringefficient energy transfer. The 26% change in resonance frequency is easyto detect and is in agreement with equation (2), indicating a 1.8 timesincrease in inductance. Similarly, the decay time increases by ˜35% inagreement with the prediction of equation (5) and (6) when theinductance increases. From equation (7) we can estimate that theequivalent resistance due to losses increased by approximately 25%.

From FIG. 5C we can see that the resonance frequency of the transmittercircuit increased to f˜244 kilohertz, and decay to half the amplitude isin ˜12.5 microseconds.

The two-fold decrease in decay time is an indication of power loss dueto Eddy current induced in the conductive foil by the time varyingmagnetic field produced by the current in the primary coil 220. Itshould be noted that placing an object comprising conductive materialsuch as metals would cause such power loss and result in decrease indecay time. For example, aluminum foils within a cigarette pack, a metalplate and the likes may be identified thus a foreign object. Similarly,if a device equipped with a receiver 290 is placed on transmitter 110incorrectly such that secondary coil 260 is not adequately aligned withprimary coil 220, some of the energy radiated from primary coil 260during the testing process discloses.

It should be noted that often the secondary coil 290 comprises aferromagnetic core to increase the inductive coupling with the primarycoil 220. The increase in effective inductance of the primary coil is toa large extant due to the influence of the ferromagnetic core that isnow in close proximity to the primary coil. Additionally, the loadingstate of the secondary coil may influence its effect on the resonanceparameters of the primary coil. For example, if the secondary coil 290is shorted, or resistively loaded, it may contribute to losses (andapparent increase in resistance) in the primary coil 220. This is causedby energy transferred from (and thus lost to) the primary coil,similarly to the effect of placing a conductive foil or a metallicobject near the primary coil, as seen in FIG. 5C.

FIG. 6A schematically depicts a block diagram of the main elements of aninductive power transfer system which uses a series resistor duringdetection of inductive coupling according to an exemplary embodiment ofthe current invention.

Voltages developed on primary coil 220 may be high during the pulseexcitation and during power transmission to the receiver 290. Forexample, voltages of tens of volts were measured during the experimentsdisclosed in FIGS. 4 and 5A-C.

In some embodiments front end is protected, for example by AC coupling,signal attenuation, voltage restriction such as Zener Diodes, and thelikes.

In other embodiments, front end 170 is disconnected during powertransmission, and pulse excitation for the purpose of detection of theinductive coupling is performed at reduced voltage of power supply 240.Reducing the voltage also reduces the power needed for such detection.According to an exemplary embodiment depicted in FIG. 6A, a resistor 610is placed in series to primary coil 612. The value of resistor 610 isset to be small as not to degrade the resonance properties of thecircuit, and to produce detectable signal in response to the currentflowing through the coil. An optional bypass switch 612 may be closedduring power transmission. Closing bypass switch 612 serves to protectfront end 170 and to eliminate power loss in the resistor during powertransmission.

It should be noted that other points in the circuit may be used by frontend 170 for probing voltages. For example, the voltage on capacitor C2may be probed by measuring the voltage between its terminal 630 andground 632. Other points may also be similarly used.

FIG. 6B schematically depicts a block diagram of the main elements of aninductive power transfer system which uses a probing coil 520 fordetection of inductive coupling according to an exemplary embodiment ofthe current invention.

Optionally the probing coil 520 is a small auxiliary coil placed near orover the primary coil 220 such that it picks up some of theelectromagnetic radiation from the primary coil. Probing coil 520 isused as a pickup device and the signals on it follow the changes incurrent at the primary coil 220.

Front end 170 is used for transforming analog signals indicative of thebehavior of the resonant circuit to digital signals to be used byprocessor 150 in order to determine proper inductive coupling betweenprimary coil 220 and secondary coil.

In one example, front end 170 comprises an Analog to Digital Converter(ADC). Since the frequency range of the typical signals is in the tensto hundreds kilohertz range, the sampling rate should be high enough toavoid aliasing. However, if the resonance frequency is to be determined,accuracy (number of bits) may be limited, for example an 8-bit ADC maybe used. Front end may further comprise a Digital Signal Processor(DSP), or a processor 150 may be used for analyzing the ADC results.

For calculating the resonance frequency, a limited number of signalcycles may be digitized, thus limiting the amount of data that has to bestored and processed. For example, zero crossing of a sinusoidal signalmay be accurately assessed from data fitting and interpolation. Bytiming at least two and optionally a few zero crossings, the frequencymay be accurately estimated. Alternatively, computation-efficient FFTand DFT (Fast Fourier Transform and Digital Fourier Transform)algorithms may be used to determine the resonant frequency,

For estimating the decay parameter of the signal, the peaks (or area) ofat least two, and optionally a few signal cycles needs to be measured.Preferably, the measured cycles are separated in time such thatamplitude difference between them is substantial.

It should be noted, that optionally several types of receivers 290 maybe designed and operate with same type of transmitter 110. Each of suchreceiver types may be associated with a change in the resonanceparameters of the primary coil which may be characteristic. Thus, theprocessor 150 may compare the assessed parameters with a list of allowedparameters. It should also be noted that each set of parameters in thelist may be associated with (optionally different) acceptance range.Thus, processor 150 may need only to establish one of the following:

-   -   No change detected in the resonance parameters of the primary        coil. In this case—continue monitoring.    -   A change was detected in the resonance parameters of the primary        coil, and the new parameters match coupling to a valid receiver.        In this case—start power transmission.    -   A change was detected in the resonance parameters of the primary        coil, and the new parameters do not match coupling to a valid        receiver. In this case—Optionally issue warning to user, and        continue monitoring.

Since the number of valid resonance frequencies is limited to theresonance frequency of uncoupled primary coil plus the frequenciesassociated with each of valid receiver type, a set of tuned filtered maybe used in order to identify the resonance parameters of the primaryfilter.

For example, a digital filter may be used. Such filters are known in theart and may be found for example inhttp://en.wikipedia.org/wiki/Digital_filter and in the Numerical Recipesseries of books on algorithms and numerical analysis by William H. etal.

Alternatively, a set of analog filters may be used. For example a set ofRLC circuits, each tuned to one specific resonance frequency, or asingle circuit, tuned each time to a different frequency, for example bymeans of a variable capacitor, or a set of capacitors.

Other analog tuned circuits may be used such as Surface Acoustic Wave(SAW) devices.

An advantage of using tuned filters is that their output has reducednoise, and thus more accurate measurements may be performed. Forexample, filtered signals may be amplified without risk of overloading,and may be used for more accurate determination of the decay envelope ofthe signal.

Alternatively a lock-in amplifier (also known as a phase-sensitivedetector or coherent detection) may be used, tuned to the specificexpected frequencies, or with swept frequencies.

In some embodiments of the invention, front end 170 comprises at leastone analog signal processing unit or function that reduces thecomputation requirement. For example, a zero crossing detector may beused to produce digital signal indicative of the times in which thedetected signal crosses the zero level. By measuring the time intervalbetween zero crossings, the resonance frequency may be determined. Aperson skilled in the art of electronics may design other means ofanalog signal processing such as Schmitt trigger and Time-to-voltageconverter, all of which may be obtained commercially.

Similarly, decay parameter of the signal may be assisted by analogsignal processing such as a peak detector which allows using a slow ADC,and one digitizing sample for obtaining the peak voltage of a signalcycle. Alternatively, the signal may be rectified and low-pass filteredto create a monotonic smooth representation of the envelope of thesignal.

In an exemplary embodiment, two different threshold levels are used inan analog comparator to determine the times these levels were crossed.The damping parameter may be determined by measuring the time differencebetween the last time the high threshold level was crossed and the lasttime the lower threshold level was crossed.

It should be noted that when two parameters of the inductive couplingare measured (frequency and decay) the identification of the properalignment of the secondary coil 260 is reliably determined Thus, theprobability of turning on power transmission when a foreign object isplaced on the transmitter is reduced. For example, a magnetic switchused as sensor 160 may be activated by an object having magneticproperties such as a refrigerator magnet which may be heated, melt orcase damage if exposed to electromagnetic radiation emitted from thetransmitter.

The current invention may provide cost saving in manufacturing thetransmitter 110 as it saves the necessity for providing sensor 160. Itshould be noted that the most or all the components needed for theoperation of invention may already exist in the transmitter and are usedfor controlling the transmitter during power transmission.

Additionally, the current invention may provide cost saving in designingand manufacturing the receiver 290. For example, already designed andused receivers may be used without modification by transmittersaccording to the current invention.

Additionally, the current invention requires no transmission of signalfrom the receiver in order for it to be detected. This saves thenecessity for providing any active or passive means in receiver 290 inorder to be detected by sensor 160. Additionally, placement of receiver290 may be reliably detected, and power transmission may commence evenif batteries in receiver 290 are completely depleted such that it cannotestablish data communication with transmitter 110 until at least somepower was received by receiver 290.

In FIG. 4, a wide square excitation pulse was used. It should be notedthat optionally a narrow pulse, having a width comparable or evennarrower that one cycle of the resonance frequency may be used.Waveforms other than rectangular pulse may be used for excitation.

In another embodiment of the current invention, excitation is in a formof alternating signal, and the amplitude and optionally the amplitudeand/or phase response of the resonance circuit is measured in order todetermine the resonance frequency and optionally the decay parameter.

For example, excitation may be in the form of weak AC (square,sinusoidal, or other) signal at various frequencies. In an exemplaryembodiment, the excitation is frequency-swept over the range offrequencies including the resonance frequencies of coupled and uncoupledprimary coil, while the response is measured. The peak responseindicates the actual resonance frequency. Damping factor α may beestimated from the value of the maximum response as well as from thefrequency width of the peak.

In another embodiment, several alignment coils it may be used toquantify the alignment of secondary coil 260 relative to the primarycoil 220. For example three or more alignment coils may be used forproviding information on the direction of misalignment.

FIG. 7 schematically depicts a spatial positioning of a primary coil 220and three alignment coils 720 a, 720 b and 720 c according to anexemplary embodiment of the current invention.

When a secondary coil 290 is misaligned with primary coil 220, itscoupling to alignment coils 720 a, 720 b and 720 c is unequal. bycomparing the coupling of secondary coil 290 to each of the threealignment coils, the direction and possibly the amount of misalignmentmay be determined.

FIG. 8 schematically depicts a method 800 for triggering powertransmission from a transmitter 110 to receiver 290 according to anexemplary embodiment of the current invention.

Transmitter 110 enters an idle state 802 when it is turned on, or afterpower transmission terminates. In this state, most of the time thetransmitter 110 is in “wait” mode 810, where power consumption may be atminimum required for operating an internal clock. After a predefinedduration, (for example every 1 second, however, shorter or longer may beused), primary coil 220 is excited 812. Resonance frequency isdetermined 814, and optionally, the decay time is determined 816. Instep 822, the effective inductance, and optionally the effectiveresistance in the primary coil circuit are assessed and compared withthe predicted, measured or otherwise known parameters indicative of aprimary coil free of influences external to the transmitter 110. If nochange is observed in the assessed parameters, the transmitter stays inidle state 802 and wait step 810 is repeated.

If change or changes are observed in the assessed parameters, theparameters are compared 834 with the predicted, measured or otherwiseknown parameters indicative of a correct placement and alignment of areceiver 290. If the assessed parameters are matched with parametersindicative of a well places receiver 290 which is designed to be coupledwith transmitter 110 power transmission starts 840. Optionally, thetransmission frequency may be selected according to the determinedresonance frequency. Where appropriate, the transmission frequency maybe selected to be approximately equal to the resonant frequency.Alternatively a transmission frequency above or below the resonantfrequency of the system may be selected, as described hereinbelow.

It should be noted, that optionally several types of receivers 290 maybe designed and operate with same type of transmitter 110. Thus,processor 150 may compare the assessed parameters with a list of allowedparameters. It also should be noted that each set of parameters in thelist may be associated with (optionally different) acceptance range.Additionally, processor 150 may identify the type of receiver used bymatching the assessed parameters to the parameters in the list andadjust the parameters used for power transmission 840. Additionally,transmitter 110 may attempt to establish communication with the receiver290, optionally based on the identification of the type of receiver 290.

Reference is now made to FIG. 9 showing a block diagram of the mainelements of an inductive power transfer system 1100 adapted to transmitpower at a non-resonant frequency according to another embodiment of theinvention. The inductive power transfer system 1100 consists of aninductive power outlet 1200 configured to provide power to a remotesecondary unit 1300. The inductive power outlet 1200 includes a primaryinductive coil 1220 wired to a power source 1240 via a driver 1230. Thedriver 1230 is configured to provide an oscillating driving voltage tothe primary inductive coil 1220.

The secondary unit 1300 includes a secondary inductive coil 1320, wiredto an electric load 1340, which is inductively coupled to the primaryinductive coil 1220. The electric load 1340 draws power from the powersource 1240. A communication channel 1120 may be provided between atransmitter 1122 associated with the secondary unit 1300 and a receiver1124 associated with the inductive power outlet 1200. The communicationchannel 1120 may provide feedback signals S and the like to the driver1230.

In some embodiments, a voltage peak detector 1140 is provided to detectlarge increases in the transmission voltage. As will be descried belowthe peak detector 1140 may be used to detect irregularities such as theremoval of the secondary unit 1200, the introduction of power drains,short circuits or the like.

FIG. 10 is a graph showing how the amplitude of the operational voltageof an inductive power transfer system varies according to thetransmission frequency. It is noted that the voltage is at its highestwhen the transmission frequency is equal to the resonant frequency f_(R)of the system, this maximum amplitude is known as the resonance peak 2.It is further noted that the slope of the graph is steepest in theregions 4 a, 4 b to either side of the resonance peak 2. Thus ininductive transfer systems, which operate at or around resonance, asmall variation in frequency results in a large change in inducedvoltage. Similarly, a small change in the resonant frequency of thesystem results in a large change in the induced voltage. For this reasonprior art resonant inductive transfer systems are typically verysensitive to small fluctuations in environmental conditions orvariations in alignment between the induction coils.

It is a particular feature of embodiments of the current invention thatthe driver 1230 (FIG. 9) is configured and operable to transmit adriving voltage which oscillates at a transmission frequency which issubstantially different from the resonant frequency of the inductivecouple. Preferably the transmission frequency is selected to lie withinone of the near-linear regions 6, 8 where the slope of thefrequency-amplitude graph is less steep.

One advantage of this embodiment of the present invention may bedemonstrated with reference now to FIG. 11. A schematic diagram is shownrepresenting a laptop computer 1340 drawing power from an inductivepower outlet 1200 via a secondary power receiving unit 1300. The powerreceiving unit 1300 includes a secondary inductive coil 1320 which isaligned to a primary inductive coil 1220 in the inductive power outlet1200. Any lateral displacement of the secondary power receiving unit1300 changes the alignment between the secondary inductive coil 1320 tothe primary inductive coil 1220. As a result of the changing alignment,the combined inductance of the coil pair changes which in turn changesthe resonant frequency of the system.

If the inductive power outlet 1200 transmits power at the resonantfrequency of the system, even a small lateral movement would reducesignificantly the amplitude of the induced voltage. In contradistinctionto the prior art, in embodiments of the present invention the inductivepower outlet 1200 transmits power at a frequency in one of the regions6, 8 to either side of the resonance peak 2 (FIG. 10) where the slope ofthe resonance graph is much shallower. Consequently, the system has amuch larger tolerance of variations such as lateral movement.

A further feature of embodiments of inductive power outlets transmittingat frequencies above the natural resonant frequency of the system isthat if the resonant frequency of the system increases for some reasons,then the transmission voltage increases sharply. In preferredembodiments, a peak detector 1140 (FIG. 9) is be provided to monitor thetransmission voltage of the power outlet 1200 and is configured todetect large increases in the transmission voltage indicating anincrease in resonant frequency.

Referring again to the resonant formula for inductive systems,

${f_{R} = \frac{1}{2\pi \sqrt{LC}}},$

it is noted that any decrease in either the inductance L or thecapacitance C of the system increases the resonant frequency and may bedetected by the peak detector 1140. Similarly, from the formula fordamped resonance (1) above, as the resistance increases, the effectiveresonant frequency increases.

As an example of the use of a peak detector 1140, reference is againmade to FIG. 11. It will be appreciated that in a desktop environment,conductive bodies such as a paper clip, metal rule, the metal casing astapler, a hole-punch or any metallic objects may be introduced betweenthe inductive power outlet 1200 and the secondary power receiving unit1300. The oscillating magnetic field produced by the primary coil 1220would then produce eddy currents in the conductive body heating it andthereby draining power from the primary coil 1220. Such a power drainmay be wasteful and/or dangerous. Power drains such as described abovegenerally reduce the inductance L of the system thereby increasing itsresonant frequency.

The inductance L of the system may also be reduced by the removal of thesecondary coil 1220, the creation of a short circuit or the like. A peakdetector 1140, wired to the inductive power outlet, may detect any ofthese scenarios as a large increase in transmission voltage. Whererequired, the power transfer system may be further configured to shutdown, issue a warning or otherwise protect the user and the system inthe event that the peak detector 1140 detects such an increase intransmission voltage.

FIG. 12 is a circuit diagram of an inductive power outlet 6200 andsecondary unit 6300. The secondary unit 6300 comprises a secondary coil6320 wired to an electric load 6340 via a rectifier 6330.

The inductive power outlet 6200 comprises a primary coil 6220 driven bya half-bridge converter 6230 connected to a power source 6240. Thehalf-bridge converter 6230 is configured to drive the primary coil 6220at a frequency higher than the resonant frequency of the system and apeak detector 6140 is configured to detect increases in the transmissionvoltage.

Although only a half-bridge converter is represented in FIG. 12, it isnoted that other possible driving circuits include: a DC-to-DCconverter, an AC-to-DC converter, an AC-to-AC converter, a flybacktransformer, a full-bridge converter, a flyback converter or a forwardconverter for example.

Another advantage of non-resonant transmission is that the transmissionfrequency may be used to regulate power transfer. Prior art inductivepower transfer systems typically regulate power transfer by altering theduty cycle of the transmission voltage. Unlike prior art systems,because embodiments of the present invention transmit at a frequency notequal to the resonant frequency of the system, the driver 1230 may beconfigured to regulate power transfer by adjusting the transmissionfrequency.

The regulation is illustrated with reference to FIG. 10. In embodimentsof the invention, the frequency of transmission may be selected to be inthe approximately linear region 8 of the curve between a lower frequencyvalue of f_(L) and an upper frequency value of f_(U). A transmissionfrequency f_(t), higher than the resonant frequency f_(R) of the system,produces an induced voltage of V_(t). The induced voltage can beincreased by reducing the transmission frequency so that it is closer tothe resonant frequency f_(R). conversely, the induced voltage may bereduced by increasing the transmission frequency so that it is furtherfrom the resonant frequency f_(R). For example, an adjustment oftransmission frequency by δf produces a change in induced voltage of δV.

In some embodiments, a communication channel 1120 (FIG. 9) is providedbetween the secondary unit 1300 and the inductive power outlet 1200 tocommunicate the required operating parameters. In embodiments of theinvention operating parameters the communication channel 1120 may beused to indicate the transmission frequency required by the electricload 1340 to the driver 1230.

The communication channel 1120 may further provide a feedback signalduring power transmission. The feedback transmission may communicaterequired or monitored operating parameters of the electric load 1240such as:

-   -   required operating voltage, current, temperature or power for        the electric load 1240,    -   the measured voltage, current, temperature or power supplied to        the electric load 1240 during operation,    -   the measured voltage, current, temperature or power received by        the electric load 1240 during operation and the like.

In some embodiments, a microcontroller in the driver 1230 may use suchfeedback parameters to calculate the required transmission frequency andto adjust the driver accordingly. Alternatively, simple feedback signalsmay be provided indicating whether more or less power is required.

One example of a power regulation method using simple feedback signalsis shown in the flowchart of FIG. 13. The method involves the followingsteps:

-   Step (a)—The driver 1230 provides an oscillating voltage at a    transmission frequency f_(t) which is higher than the resonant    frequency f_(R) of the system.-   Step (b)—A secondary voltage is induced in the secondary coil 1320.-   Step (c)—A power monitor in the secondary unit 1300, monitors the    power received by the electric load 1340.-   Step (d)—If the power received by the electric load 1340 lies within    a predetermined range then no action is taken. If the power received    by the electric load 1340 is below the predetermined range, then a    feedback signal of a first type S_(a) is sent to the driver. If the    power received by the electric load 1340 is above the predetermined    range, then a feedback signal of a second type S_(b) is sent to the    driver.-   Step (e)—A feedback signal is received by the driver 1230.-   Step (f)—If the received feedback signal is of the first type S_(a),    then the transmission frequency is increased by an incremental value    +δf₁. If the received feedback signal is of the second type S_(b),    then the transmission frequency is decreased by an incremental value    −δf₂.

It is noted that by using the power regulation method described above,when the power received by the load is too high, a series of feedbacksignals of the first type S_(a) will be transmitted until the power isreduced into the acceptable range. Likewise when the power received bythe load is too low, a series of feedback signals of the second typeS_(b) will be transmitted until the power is increased into theacceptable range. It is noted that the positive incremental value δf₁may be greater than, less than or equal to the negative incrementalvalue δf₂.

Alternatively, other power regulation methods using frequency adjustmentmay be used. For example, the operating parameters of the electric loadmay be monitored and their values may be transmitted to the power outletvia the communications channel 1120. A processor in the power outlet maythen calculate the required transmission frequency directly.

The method described hereinabove, refers to a non-resonant transmissionfrequency lying within the linear region 8 (FIG. 10), higher than theresonant peak 2. It will be appreciated however that in alternativeembodiments frequency-controlled power regulation may be achieved whenthe transmission frequency lies in the lower linear region of theresonance curve. Nevertheless, for certain embodiments, the selection oftransmission frequencies in the higher linear 8 may be preferred,particularly where peak detection, as described above, is required.

Referring back to FIG. 9, various transmitters 1122 and receivers 1124may be used for the communication channel 1120. Where, as is often thecase for inductive systems, the primary and secondary coils 1220, 1320are galvanically isolated optocouplers, for example, may be used. Alight emitting diode serves as a transmitter and sends encoded opticalsignals over short distances to a photo-transistor which serves as areceiver. However, optocouplers typically need to be aligned such thatthere is a line-of-sight between transmitter and receiver. In systemswhere alignment between the transmitter and receiver may be difficult toachieve, optocoupling may be inappropriate and alternative systems maybe preferred such as ultrasonic signals transmitted by piezoelectricelements or radio signals such as Bluetooth, Wi-Fi and the like.Alternatively the primary and secondary coils 1220, 1320 may themselvesserve as the transmitter 1122 and receiver 1124.

In certain embodiments, an optical transmitter, such as a light emittingdiode (LED) for example, is incorporated within the secondary unit 1300and is configured and operable to transmit electromagnetic radiation ofa type and intensity capable of penetrating the casings of both thesecondary unit 1300, and the power outlet 1200. An optical receiver,such as a photodiode, a phototransistor, a light dependent resistors ofthe like, is incorporated within the power outlet 1200 for receiving theelectromagnetic radiation.

Reference to the block diagram of FIG. 14, it is a particular feature ofcertain embodiments of the invention that an inductive communicationschannel 2120 is incorporated into the inductive power transfer system2100 for transferring signals between a inductive power outlet 2200 anda remote secondary unit 2300. The communication channel 2120 isconfigured to produce an output signal S_(out) in the power outlet 2200when an input signal S_(in) is provided by the secondary unit 2300without interrupting the inductive power transfer from the outlet 2200to the secondary unit 2300.

The inductive power outlet 2200 includes a primary inductive coil 2220wired to a power source 2240 via a driver 2230. The driver 2230 isconfigured to provide an oscillating driving voltage to the primaryinductive coil 2220, typically at a voltage transmission frequency f_(t)which is higher than the resonant frequency f_(R) of the system.

The secondary unit 2300 includes a secondary inductive coil 2320, wiredto an electric load 2340, which is inductively coupled to the primaryinductive coil 2220. The electric load 2340 draws power from the powersource 2240. Where the electric load 2340 requires a direct currentsupply, for example a charging device for an electrochemical cell or thelike, a rectifier 2330 may be provided to rectify the alternatingcurrent signal induced in the secondary coil 2320.

An inductive communication channel 2120 is provided for transferringsignals from the secondary inductive coil 2320 to the primary inductivecoil 2220 concurrently with uninterrupted inductive power transfer fromthe primary inductive coil 2220 to the secondary inductive coil 2320.The communication channel 2120 may provide feedback signals to thedriver 2230.

The inductive communication channel 2120 includes a transmission circuit2122 and a receiving circuit 2124. The transmission circuit 2122 iswired to the secondary coil 2320, optionally via a rectifier 2330, andthe receiving circuit 2124 is wired to the primary coil 2220.

The signal transmission circuit 2122 includes at least one electricalelement 2126, selected such that when it is connected to the secondarycoil 2320, the resonant frequency f_(R) of the system increases. Thetransmission circuit 2122 is configured to selectively connect theelectrical element 2126 to the secondary coil 2320. As noted above, anydecrease in either the inductance L or the capacitance C increases theresonant frequency of the system. Optionally, the electrical element2126 may be have a low resistance for example, with a resistance sayunder 50 ohms and preferably about 1 ohm.

Typically, the signal receiving circuit 2124 includes a voltage peakdetector 2128 configured to detect large increases in the transmissionvoltage. In systems where the voltage transmission frequency f_(t) ishigher than the resonant frequency f_(R) of the system, such largeincreases in transmission voltage may be caused by an increase in theresonant frequency f_(R) thereby indicating that the electrical element2126 has been connected to the secondary coil 2320. Thus thetransmission circuit 2122 may be used to send a signal pulse to thereceiving circuit 2124 and a coded signal may be constructed from suchpulses.

According to some embodiments, the transmission circuit 2122 may alsoinclude a modulator (not shown) for modulating a bit-rate signal withthe input signal S_(in). The electrical element 2126 may then beconnected to the secondary inductive coil 2320 according to themodulated signal. The receiving circuit 2124 may include a demodulator(not shown) for demodulating the modulated signal. For example thevoltage peak detector 2128 may be connected to a correlator forcross-correlating the amplitude of the primary voltage with the bit-ratesignal thereby producing the output signal S_(out).

In other embodiments, a plurality of electrical elements 2126 may beprovided which may be selectively connected to induce a plurality ofvoltage peaks of varying sizes in the amplitude of the primary voltage.The size of the voltage peak detected by the peak detector 2128 may beused to transfer multiple signals.

FIG. 15A is a graph showing how the amplitude of the operational voltagevaries according to the transmission frequency. It is noted that thevoltage is at its highest when the transmission frequency is equal tothe resonant frequency f_(R) of the system, this maximum amplitude isknown as the resonance peak 2. If the resonant frequency f_(R) of thesystem increases, a new resonance peak 2′ is produced.

FIG. 15B is a graph showing how the amplitude of the operational voltagevaries according to the transmission frequency for damped system. It isnoted that in damped systems, for example where a resistor is introducedinto the circuit, the resonance curve is shifted and a new effectiveresonance peak 2″ is produced.

Accordingly, where an inductive power transfer system 2100 may beconfigured to operate at a given transmission frequency f_(t) higherthan the resonant frequency f_(R) of the system. The normal operatingvoltage V_(t) of such a system may be monitored by the voltage peakdetector 2128. When the electric element 2126 is connected to thesecondary inductive coil 2320 the resonant frequency of the systemincreases, either due to a decrease in inductance, a decrease incapacitance or through damping effects of an increase in resistance.Therefore, the operating voltage increases to a higher value V_(t)′,V_(t)″. This increase is detected by the voltage peak detector 2128.

It is noted that in contradistinction to prior art inductive signaltransfer systems such as described in U.S. Pat. No. 5,455,466 to TerryJ. Parks and David S. Register, the present invention enables datasignals to be transferred from the secondary coil 2320 to the primarycoil 2220 concurrently with inductive transfer of power from the primarycoil 2220 to the secondary coil 2320. Consequently, the signal transfersystem may be used to provide feedback signals for real time powerregulation.

FIG. 16A shows an exemplary circuit diagram of an inductive power outlet7200 and a secondary unit 7300, according to another embodiment of theinvention. An inductive feedback channel 7120 is provided fortransferring signals between the coils concurrently with uninterruptedinductive power transfer.

The inductive power outlet 7200 comprises a primary coil 7220 driven bya half-bridge converter 7230 connected to a power source 7240. Thehalf-bridge converter 7230 is configured to drive the primary coil 7220at a frequency higher than the resonant frequency of the system. Thesecondary unit 7300 comprises a secondary coil 7320 wired to the inputterminals T₁, T₂ of a rectifier 7330, and an electric load 7340 wired tothe output terminals T₃, T₄ of the rectifier 7330.

The inductive feedback channel 7120 comprises a transmission circuit7122, in the secondary unit 7300 and a receiving circuit 7124 in theinductive power outlet 7200. The transmission circuit 7122 comprises anelectrical resistor 7126 connected to the rectifier 7330 via a powerMOSFET switch 7125. A modulator 7123 may provide an input signal S_(in)to the power MOSFET 7125.

It is noted that in this embodiment the transmission circuit 7122 iswired to one input terminal T₁ and one output terminal T₃ of therectifier 7330. This configuration is particularly advantageous as, evenwhen the transmission circuit 7122 is connected; the resistor 7126 onlydraws power from the system during one half of the AC cycle, therebysignificantly reducing power loss.

The receiving circuit 7124 includes a voltage peak detector 7128 that isconfigured to detect increases in the transmission voltage, and ademodulator 7129 for producing an output signal S_(out).

Referring now to FIG. 16B, is another circuit diagram exemplary showingan inductive power outlet 7200′ and a secondary unit 7300, according toan alternative embodiment of the invention. An alternative inductivefeedback channel 7120′ may include an additional pickup inductor L3 forreceiving feedback signals. It is noted that such a pickup coil maycomprise an auxiliary coil 8220 which is not electrically connected tothe power transmission circuit.

The pickup coil may serve as a magnetic probe operable to detectfluctuations in the magnetic field in the vicinity of the primary andsecondary inductors of the inductive couple. The magnetic probe may beconnected to a signal detector such as a receiving circuit configured todetect feedback signals encoded in the magnetic fluctuations. The signaldetector may be configured to produce an output signal which may be usedto provide operating feedback to the driving circuit, for example byproviding instructions to adjust transmission parameters for example toshift transmission frequency up or down to adjust output power.Alternatively, the feedback signals may be used communicate other datasuch as operating parameters from the inductive receiver to theinductive transmitter.

Where required one or more auxiliary coils 8220 may be incorporated intoan inductive power transmitter, possibly adjacent to the primaryinductive coil and operable to pick up feedback signals.

Accordingly, data generated in the inductive power receiver may bedecrypted in the inductive power transmitter. In some embodiments, theuse of an external (external to the power path) coil may provide betterfiltering. For example, the independent coil may be connected on bothsides simultaneously to some filter whereas the power path may haverestricted connections.

Furthermore the external coil may allow filtering of finer parameters,such as operating frequency even before power transfer and perhapsallowing more levels of data etc.

It may also be possible to use multiple pickup coils and thus allow thecapturing of data when the inductive power receiver is laterallymisaligned. Different coils can pick up data from different locations.This may improve data channel at misalignment. The different data fromthe different coils can be added by some analog circuits.

With reference now to FIG. 17, a flowchart is presented showing the mainsteps in a method for transferring a signal from the secondary inductivecoil to a primary inductive coil of an inductive power transfer system.The method includes the following steps:

-   Step (i)—connecting the primary inductive coil to a voltage monitor    for monitoring the amplitude of a primary voltage across the primary    coil;-   Step (ii)—connecting the secondary inductive coil to a transmission    circuit for selectively increasing the resonant frequency of the    inductive power transfer system;-   Step (iii)—providing an oscillating voltage to the primary inductive    coil at an initial transmission frequency higher than the resonant    frequency thereby inducing a voltage in the secondary inductive    coil;-   Step (iv)—using the transmission circuit to modulate a bit-rate    signal with the input signal to create a modulated signal and    connecting the electrical element to the secondary inductive coil    intermittently according to the modulated signal, and-   Step (v)—using the voltage monitor to cross-correlate the amplitude    of the primary voltage with the bit-rate signal for producing an    output signal.

Therefore, the inductive communication channel 2120 may be used totransfer a feedback signal from the secondary inductive coil to theprimary inductive coil for regulating power transfer across an inductivepower coupling as described above.

It will be appreciated that embodiments of the present invention may beuseful in a wide range of applications. Inductive power receivers may beused to wirelessly provide power for a variety of electrical devices.Embodiments of the present invention may be integrated into suchinductive power receivers. In particular, because non-resonanttransmission uses lower transmission voltages, heat loss from thenon-resonant system is lower. Thus embodiments of the current inventionmay be of particular use when incorporated within high powerapplications such as power tools, kitchen appliances, bathroomappliances, computers, media players, office equipment and the like.

The reduced heat loss, associated with embodiments of the non-resonantsystems of the invention, is particularly useful when heat dissipationis difficult for example when power receiver has small dimensions or forheat-sensitive equipment such as measuring devices. Also, it isdesirable that devices implanted into a living body do not dissipatelarge amounts of heat into the body. Therefore, non-resonant inductivetransfer is well suited to implanted devices, such as pace makers,trackers and the like.

It is also noted that in recent years public concern about the threat ofa global energy crisis has resulted in a greater emphasis being placedupon optimizing the efficiency of energy transfer. It is difficult toachieve more demanding specifications using existing technology and, inthis context, embodiments of the present invention may be used toprovide high powers with very low energy losses. Consequently thecurrent invention is an important element in the drive for greaterefficiency.

Furthermore embodiments of the present invention may be advantageouslyutilized in inductive power transfer systems in any of the variousapplications in which power is transferred from a primary coil to aremote secondary coil. Amongst others, such applications include:

-   -   inductive chargers for use charging electronic devices,    -   inductive power adaptors for powering electronic devices such as        computers, televisions, kitchen appliances, office equipment and        the like,    -   medical applications in which power is transferred remotely to        devices implanted in a patient,    -   communications with remote RFID tags,    -   military application in which power is transferred across thick        armored plating,    -   communication or inductive energy transfer to secondary        inductive coils buried underground.    -   communication or inductive energy transfer to secondary        inductive coils submerged under water, for example in submarine        applications, and    -   communication or inductive energy with secondary coils which are        moving relative to the primary coil.

Thus, by using a transmission voltage oscillating at a frequencydifferent from the resonant frequency of the system, the inductivetransfer system has a higher tolerance to environmental fluctuations andvariations in inductive coil alignment than other transfer systems andthe frequency may be used to regulate power transfer. Moreover, when thetransmission frequency is higher than the resonant frequency of thesystem, a peak detector may be used to indicate hazards and provide aninductive communication channel.

When two parameters are used (that is the resonance frequency and decayvalues or their equivalent, and their derived parameters the inductanceand resistance), each receiver type may be associated with a rangewithin the two dimensional parameter space. Thus, the chances ofmistakenly identifying a foreign object or a defective, altered ormisaligned reviver as a well-placed are reduced.

Optionally, if the assessed parameters ware found to change, but differfrom any parameters set in the list, a warning signal may be issued 830.Warning signal may be an audio signal such as a beep or recordedmessage, or a visual signal such as a light or a note on a display, or acombination of audio and visual signals. When alignment coils such ascoils 720 a-c are used, warning signal may include alignmentinstructions.

As used herein, the term “processor”, “computer” or “module” may includeany processor-based or microprocessor-based system including systemsusing microcontrollers, reduced instruction set computers (RISC),application specific integrated circuits (ASICs), logic circuits, andany other circuit or processor capable of executing the functionsdescribed herein. The above examples are exemplary only, and are thusnot intended to limit in any way the definition and/or meaning of theterm “computer”.

The computer or processor executes a set of instructions that are storedin one or more storage elements, in order to process input data. The setof instructions may include various commands that instruct the computeror processor as a processing machine to perform specific operations suchas the methods and processes of the various embodiments of theinvention. The set of instructions may be in the form of a softwareprogram. The software may be in various forms such as system software orapplication software. Further, the software may be in the form of acollection of separate programs or modules, a program module within alarger program or a portion of a program module. The software also mayinclude modular programming in the form of object-oriented programming.The processing of input data by the processing machine may be inresponse to operator commands, or in response to results of previousprocessing, or in response to a request made by another processingmachine.

As used herein, the terms “software” and “firmware” are interchangeable,and include any computer program stored in memory for execution by acomputer, including RAM memory, ROM memory, EPROM memory, EEPROM memory,and non-volatile RAM (NVRAM) memory. The above memory types areexemplary only, and are thus not limiting as to the types of memoryusable for storage of a computer program.

It is to be understood that the above description is intended to beillustrative, and not restrictive. For example, the above-describedembodiments (and/or aspects thereof) may be used in combination witheach other. In addition, many modifications may be made to adapt aparticular situation or material to the teachings of the variousembodiments of the invention without departing from their scope. Whilethe dimensions and types of materials described herein are intended todefine the parameters of the various embodiments of the invention, theembodiments are by no means limiting and are exemplary embodiments. Manyother embodiments will be apparent to those of skill in the art uponreviewing the above description. The scope of the various embodiments ofthe invention should, therefore, be determined with reference to theappended claims, along with the full scope of equivalents to which suchclaims are entitled. In the appended claims, the terms “including” and“in which” are used as the plain-English equivalents of the respectiveterms “comprising” and “wherein.” Moreover, in the following claims, theterms “first,” “second,” and “third,” etc. are used merely as labels,and are not intended to impose numerical requirements on their objects.

Further, the limitations of the following claims are not written inmeans-plus-function format and are not intended to be interpreted basedon 35 U.S.C. §112, sixth paragraph, unless and until such claimlimitations expressly use the phrase “means for” followed by a statementof function void of further structure.

This written description uses examples to disclose the variousembodiments of the disclosure, including the best mode, and also toenable any person skilled in the art to practice the various embodimentsof the disclosure, including making and using any devices or systems andperforming any incorporated methods. The patentable scope of the variousembodiments of the disclosure is defined by the claims, and may includeother examples that occur to those skilled in the art. Such otherexamples are intended to be within the scope of the claims if theexamples have structural elements that do not differ from the literallanguage of the claims, or if the examples include equivalent structuralelements with insubstantial differences from the literal languages ofthe claims.

Although the invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives, modificationsand variations will be apparent to those skilled in the art.Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and broad scopeof the appended claims. All publications, patents and patentapplications mentioned in this specification are herein incorporated intheir entirety by reference into the specification, to the same extentas if each individual publication, patent or patent application wasspecifically and individually indicated to be incorporated herein byreference. In addition, citation or identification of any reference inthis application shall not be construed as an admission that suchreference is available as prior art to the present invention.

While exemplary embodiments are described above, it is not intended thatthese embodiments describe all possible forms of the invention. Rather,the words used in the specification are words of description rather thanlimitation, and it is understood that various changes may be madewithout departing from the spirit and scope of the invention.Additionally, the features of various implementing embodiments may becombined to form further embodiments of the disclosure.

1. A triggerable power transmitter for power transmission from a primarycoil in the power transmitter to an inductively coupled secondary coilin a power receiver comprising: a primary coil, capable of beinginductively coupled to a secondary coil in a power receiver; a driver,capable of electrically driving said primary coil; a probing coil,capable of receiving analog signals indicative of resonance propertiesof said primary coil; and a processor, capable of generating digitalinformation in response to said analog signal and capable of:determining if said primary coil is coupled to a secondary coil based onsaid digital information, and controlling said driver to transmit powerfrom said primary coil to said secondary coil when said primary coil isinductively coupled to said secondary coil.
 2. The power transmitter ofclaim 1, and comprising a resistor, placed in series to primary coil andcapable of providing said front end with analog signal indicative ofresonance properties of said primary coil.
 3. The power transmitter ofclaim 4, and comprising a switch placed in parallel to said resistor andcapable of shorting out said resistor when power is transmitted fromsaid primary coil to said secondary coil.
 4. A triggerable powertransmitter for power transmission from a primary coil in the powertransmitter to an inductively coupled secondary coil in a power receivercomprising: a primary coil, capable of being inductively coupled to asecondary coil in a power receiver; a driver, capable of electricallydriving said primary coil; a front end, capable of receiving analogsignal indicative of resonance properties of said primary coil andcapable of generating digital information in response to said analogsignal; a resistor, connectable in series with said primary coil andcapable of providing said front end with an analog signal indicative ofresonance properties of said primary coil; a switch, in parallel to saidresistor and capable of shorting out said resistor when power istransmitted from said primary coil to said secondary coil; and aprocessor, receiving said digital information and capable of:determining if said primary coil is coupled to a secondary coil based onsaid digital information, and controlling said driver to transmit powerfrom said primary coil to said secondary coil when said primary coil isinductively coupled to said secondary coil.
 5. The power transmitter ofclaim 4 further comprising at least one analog filter tuned to aspecific resonance frequency.
 6. The power transmitter of claim 4further comprising a set of RLC circuits, each tuned to one specificresonance frequency.
 7. The power transmitter of claim 4, and comprisinga probing coil, said probing coil is capable of providing said front endwith analog signal indicative of resonance properties of said primarycoil.
 8. A method of triggering power transmission in inductivelycoupled power transmission system comprising: a. waiting a timeduration; b. electrically exciting a primary coil in a powertransmitter; c. receiving signal indicative of resonance properties ofsaid primary coil; d. applying at least one tuned filter to a signalindicative of resonance properties of said primary coil therebydetermining if a secondary coil in a power receiver is inductivelycoupled to said primary coil; and e. triggering power transmission fromsaid primary coil to said secondary coil if said secondary coil isinductively coupled to said primary coil, or repeating steps a-d if saidsecondary coil is not inductively coupled to said primary coil.
 9. Themethod of triggering power transmission of claim 8, wherein saidapplying at least one tuned filter to a signal indicative of resonanceproperties of said primary coil comprises applying a plurality of analogfilters.
 10. The method of triggering power transmission of claim 8,wherein said exciting of a primary coil in a power transmitter comprisesapplying a short electric pulse to said primary coil.
 11. The method oftriggering power transmission of claim 8, wherein said determining if asecondary coil is inductively coupled to said primary coil comprisesdetermining a change in resonance frequency of said primary coil. 12.The method of triggering power transmission of claim 11, wherein saidchange in resonance frequency of said primary coil is a reduction ofsaid resonance frequency.
 13. The method of triggering powertransmission of claim 8, wherein said determining if a secondary coil isinductively coupled to said primary coil comprises determining a changein effective inductance of said primary coil.
 14. The method oftriggering power transmission of claim 8, wherein said determining if asecondary coil is inductively coupled to said primary coil furthercomprises determining a change in effective resistance of said primarycoil.
 15. The method of triggering power transmission of claim 8,wherein said determining if a secondary coil is inductively coupled tosaid primary coil comprises determining a match between: valuesindicative of effective inductance of said primary coil; and valuesindicative of effective resistance of said primary coil to at least oneset of values in a list of values associated with a primary coilinductively coupled to a plurality of different types of powerreceivers.
 16. The method of triggering power transmission of claim 15,wherein said triggering power transmission from said primary coil tosaid secondary coil if said secondary coil is inductively coupled tosaid primary coil comprises controlling the power transmission accordingto type of power receiver associated with the matched values indicativeof effective inductance of said primary coil; and values indicative ofeffective resistance of said primary coil.
 17. The method of triggeringpower transmission of claim 16, wherein said repeating steps a-d if saidsecondary coil is not inductively coupled to said primary coil furthercomprising issuing a warning if said signal indicative of resonanceproperties of said primary coil indicates that an object other than asecondary coil is inductively coupled to said primary coil.
 18. Themethod of triggering power transmission of claim 8, wherein saidexciting of a primary coil in a power transmitter comprises shortduration activation of a driver used for driving said primary coilduring power transmission from said primary coil to said secondary coil.19. The method of triggering power transmission of claim 8, wherein saidexciting of a primary coil in a power transmitter comprises activationof a driver used for driving said primary coil during power transmissionfrom said primary coil to said secondary coil at power levelsignificantly reduced compared to power levels used for driving saidprimary coil during power transmission.
 20. The method of triggeringpower transmission of claim 19, wherein: said exciting of a primary coilin a power transmitter at reduced power level comprising exciting saidprimary coil at a plurality of frequencies, and said determining if asecondary coil in a power receiver is inductively coupled to saidprimary coil comprises assessing frequency response of said primarycoil.